Why does the grease Pencil Fill becomes transparent when zoomed out
Not sure if you've figured out a solution for this problem, but if not, this could be worth a shot: 


Select Grease Pencil object
Add an "Offset" modifier
Try changing the Location value by either +0.01 or -0.01 across each axis, then render the results to see if it's fixed the issue. Do it one at a time; you're basically testing which axis the grease pencil needs to be "pushed away from the object" &amp; towards the camera on for it to display properly.


I've had a similar issue which only happens the further the object gets away from the camera. Up close is fine, then if I zoom out, I've had to use this offset modifier &amp; change the location value for my Y axis to -0.02 .. then keyframe it back to 0 when moving back in. The further away the object is, Blender seems to get confused by how close the grease pencil is to another GP object or a 3D object, hence using the offset to move it further away. 

Hope this helps